This project seeks to improve our understanding of the detailed circuitry of the thalamus and its relation to arousal, sleep-wakefulness cycles, and information transfer from subcortical relays to the cerebral cortex. In previous work on this project, light microscopic analysis of the noradrenergic (NA) input to the thalamic reticular nucleus (TRN) showed that the NA axons from a moderately dense meshwork of boutons en passant within this thalamic nucleus. The NA axons are seemingly randomly distributed within the TRN, with the dopamine-beta-hydroxylase positive (DBH+) axons appearing to about upon both proximal and distal dendritic processes of TRN neurons, and less frequently, upon somata. This year, electron microscopic examination of TRN neurophil processed for DBH immunohistochemistry indicated that DBH+ axonal boutons make atypical asymmetric synaptic contacts with reticular neuron dendrites of all diameters. These observations show that the locus coeruleus NA system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, can influence the signal flow indirectly through its projection upon TRN neurons. This project also involves an analysis of gamma-aminobutyric acid containing (GABAergic) systems in the thalamus. One model of thalamic gating mechanisms proposes a differential role of TRN terminals upon the principal relay neurons and the GABAergic local circuit neurons (LCNs) of the dorsal thalamus during different behavioral states. Much of the work upon which the model is based involves cats, but studies of thalamic function in vitro have frequently relied upon rodent preparations, such as guinea pig slices. Since there are virtually no GABAergic neurons within many relay nuclei of rats and mice, the model could be questioned in terms of its applicability to rodents such as guinea pigs. Accordingly, the distribution of dorsal thalamic LCNs was examined in guinea pigs, with immunohistochemical techniques. GABAergic neurons are more widespread within guinea pig thalamus than in the thalami of rats and mice, and a distinct population of GABA neurons is found within the guinea pig ventrobasal complex, which is not found in rats and mice. Thus, there is considerable variation in the distribution of LCNs among different rodent species, a finding which is of relevance to models of thalamic gating.